As a conventional technology, a fracture-type torque limiter which has a member or a portion to be fractured when a transmission load over a predetermined value is provided between a member at the driving side and a member at the driven side. For example, in Patent document 1, a fracture-type torque limiter is disclosed wherein, when a compressor is abnormally stopped by a failure, etc., a coupling member which is provided between a pulley at the drive source side and a rotation transmission plate attached to a shaft of the compressor at the driven side is fractured. In the conventional fracture-type torque limiter, however, because fluctuation load due to torque fluctuation entirely acts on the part or the member to be fractured, fatigue phenomenon occurs at this portion, and therefore, it may be fractured at a torque lower than a target fracture torque.
For such a problem, a power transmission, in which a drive body and a driven body rotated in the same direction are coupled through a coupling portion, is known and, by forming the coupling portion from members different from each other, the fatigue phenomenon of the part to be fractured, that has been a problem in the conventional technology, is decreased, and the target torque interruption can be securely achieved (Japanese Patent Application 2006-241277).
According to this previously proposed invention, the above-described the coupling portion is constituted by combination of a positive torque transmission member for transmitting torque in a direction of forward rotation and interrupting transmission of torque by its own fracture when the driving load of the driven body exceeds a predetermined level and a negative torque transmission member capable of transmitting torque in a direction of reverse rotation, and by providing pre-tensions to the above-described positive torque transmission member and the above-described negative torque transmission member in directions opposite to each other, namely, by providing a tensile pre-load and a compressive pre-load thereto, the tensile pre-load and the compressive pre-load can operate in directions opposite to each other when the amplitude of torque fluctuation is decreased, and the amplitude of torque fluctuation can be efficiently decreased. Therefore, by this previous proposal, the conventional problems can be solved, thereby realizing a power transmission having an excellent reliability.    Patent document 1: Japanese Utility Model 6-39105